The invention discloses a nuclear magnetic resonance sensor for detecting a composite insulator umbrella skirt. The nuclear magnetic resonance sensor comprises a shell, a magnet held in a magnet cavity, and a radio frequency coil located between an upper magnet and a lower magnet, wherein the shell comprises a first shell and a second shell in detachable splicing and fixing, the first shell and the second shell are spliced to form the magnet cavity for surrounding an umbrella skirt plate of the insulator, through holes for the penetration of a mandril of the insulator are arranged at the upper side and the lower side of the shell; the magnet body comprises the upper magnet located on the upper surface of the umbrella skirt plate and the lower magnet located on the lower surface of the umbrella skirt plate, the polarities of the opposite sides of the upper magnet and the lower magnet are on the contrary; the radio frequency coil comprises an inner coil surrounding the periphery of the mandril and an outer coil surrounding the inner coil, wherein a reserved radio frequency region is formed between the inner coil and the outer coil, and a radio frequency contact connected with the radio frequency connector is arranged on the external of the shell. Compared with the prior art, the detection process is simpler and more convenient by use of the detachable shell structure through the nuclear magnetic resonance sensor, thereby conveniently and effectively detecting the ageing state of the umbrella skirt.